The refrigerant circulating in the cooling cycle has to be realized in such a manner that it can be used in the cooling cycle within the above-mentioned temperature range. Due to legal provisions, the refrigerant must not substantially contribute to the ozone depletion in the atmosphere or to global warming. Thus, no fluorinated gases or chlorinated materials may be used as refrigerants, which is why natural refrigerants or rather gases are eligible instead. Furthermore, the refrigerant should not be flammable so that filling, shipping or operating the test chamber is not complicated due to security measures which have to be possibly met. Producing a cooling cycle also becomes more expensive when using a flammable refrigerant in consequence of required constructive measures appertaining thereto. Being flammable is understood in this context to mean the characteristic of the refrigerant to react with ambient oxygen under release of heat. A refrigerant is flammable in particular when it falls under fire class C of the European Standard EN 2 as well as under DIN 378 classes A2, A2L and A3.
Furthermore, a refrigerant should comprise a fairly low CO2 equivalent, i.e. a fairly low greenhouse potential or global warming potential (GWP) should be as low as possible in order to prevent indirectly damaging the environment via the refrigerant upon release. The GWP indicates how much a predetermined mass of a greenhouse gas contributes to global warming, with carbon dioxide serving as a comparable figure. The value describes the average warming effect over a predetermined period of time, 20 years having been stipulated for comparison. For defining the relative CO2 equivalent or GWP, the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), Assessment Report, Appendix 8.A, Table 8.A is referred to.
A disadvantage of refrigerants having a low GWP, for example <2500, is that these refrigerants comprise a partially significantly reduced refrigerating capacity in comparison to refrigerants having a comparatively high GWP in the temperature ranges relevant for test chambers.